I. Popular Understanding of WDM (Wavelength Division Multiplexing) :
Wavelength Division multiplexing (WDM) is a general term for the technology of multiplexing optical signals of different wavelengths into the same optical fiber for synchronous transmission. It is the most core and economical means of expanding optical communication.
For example, in the ITU G.695 standard, CWDM plans 18 standard wavelengths (1270nm - 1610nm) with a 20nm interval, covering five major bands: O-Band, E-Band, S+C+L-Band, ITU+1 wavelengths (1271nm, 1291nm...) are also commonly used in engineering 1611nm.
WDM Core Advantage:
Used to save fiber optic resources and reduce link matching transmission equipment and units;
Multi-channel signals facilitate unified carrying, centralized management, flexible expansion and low deployment cost.
2. Two basic functions of WDM
No matter how complex the product form and networking structure are, all WDMS are based on two types of core functions:
1. Mux/Demux (combining/splitting)
Combining (Mux) : Merging multiple light signals of different wavelengths into a single optical fiber for transmission;
Demux: A mixed wavelength signal is restored and separated at the receiving end and output to multiple receiving devices.
2. OADM (Optical Bifurcation Multiplexing)
Flexible wavelength uplink and downlink at link intermediate nodes:
DROP: Downhaul the specified wavelength signal and divert it to the local device;
ADD: Upload a new local wavelength signal and continue transmitting to the remote end.
A common analogy: In the high-speed rail transportation scenario, optical signal transmission is like a high-speed rail line (Shenzhen → Dongguan → Guangzhou) :
Concentrate passengers at the initial station, Shenzhen Station, as opposed to the Hebo Mux;
At Dongguan Station, some passengers DROP off and some new passengers ADD on → corresponding to OADM;
At the terminal station, Guangzhou Station, all passengers get off and disperse to go home → corresponding to Demux.
3. Six Typical Application Scenarios of WDM (CWDM Examples)
All application scenarios of WDM are based on the two fundamental functions of combining/splitting and OADM uplink/downlink. Based on various optical module interfaces and transmission links, WDM can be classified into six standard schemes in practical engineering applications.
The following text takes CWDM as an example. The principle of DWDM is the same. Special scenarios will be explained separately.
1. Single-fiber unidirectional transmission scheme (Duplex transceiver module)
The transceiver module is Duplex LC dual optical port, and the signal is unidirectional transmission on a single fiber;
Link: MUX on the left → 1-CHOADM in the middle → DEMUX on the right.
2 Dual-fiber bidirectional transmission scheme (Duplex transceiver module)
The transceiver module is Duplex LC dual optical port, TX/RX uses the same wavelength, dual-fiber bidirectional transmission;
Link: MUX&DEMUX integrated units at both ends → 1-CH Duplex OADM in the middle.
Question discussion: What is the theoretical maximum number of supported terminals for full-channel CWDM?
3. Single-fiber bidirectional transmission scheme (Duplex transceiver module, TX/RX different wavelengths)
The transceiver module is Duplex LC dual optical port, TX/RX uses different wavelengths, single-fiber bidirectional transmission;
The two ends are WDM MUX&DEMUX integrated units in reverse sequence (half channel transmission and half reception), in order to optimize the Uniformity of insertion loss of the link (IL Uniformity); In the middle is 1-CH BIDI OADM.
Question discussion: What is the theoretical maximum number of supported terminals for full-channel CWDM?
4 Single-fiber bidirectional transmission scheme (Duplex transceiver module, TX/RX at the same wavelength)
The transceiver module is Duplex LC dual optical port, TX/RX uses the same wavelength, single-fiber bidirectional transmission;
At both ends are MUX&DEMUX integrated units (with built-in Circulator), and in the middle is 2-CH BIDI OADM.
Question discussion: What is the theoretical maximum number of supported terminals for full-channel CWDM?
5. Single-fiber bidirectional transmission scheme (BIDI transceiver module, DWDM not recommended)
The transceiver module is BIDI single optical port, TX/RX uses different wavelengths, single-fiber bidirectional transmission;
The Dual-band architecture, with MUX&DEMUX integrated units at both ends and 2-CH BIDI dual-band OADM in the middle.
Question Discussion: What is the theoretical maximum number of supported terminals for full-channel CWDM?
6. Single-fiber bidirectional transmission scheme (SFSW BIDI transceiver module, not using DWDM)
The transceiver module is SFSW BIDI Single fiber single wavelength, single-fiber bidirectional transmission at the same wavelength;
Link: MUX on the left → 2-CH SFSW BIDI OADM in the middle → DEMUX on the right.
Question discussion: What is the theoretical maximum number of supported terminals for full-channel CWDM?
WDM technology has become a standard basic capability in scenarios such as fiber access, metropolitan area networks, data center interconnections, security monitoring, and industrial transmission, thanks to its high scalability, low unit bandwidth cost, easy deployment, and flexible networking.
In summary, from a technical layering perspective, CWDM and DWDM correspond to different application scenarios:
CWDM (Coarse Wavelength Division) : Characterized by low cost and wide wavelength intervals, it is suitable for medium and short-distance, general-purpose transmission scenarios.
DWDM (Dense Wavelength Division) : Centered on high channel density, long distance, and large bandwidth carrying capacity, it mainly serves backbone networks and core service networks.
The two are not substitutable but engineered solutions for different bandwidth densities and transmission distances.
With the rapid development of all-optical networks and computing power networks, the value of WDM continues to stand out: carrying the maximum information capacity with the minimum fiber optic resources, becoming the underlying infrastructure for high-speed, stable, and large-scale data transmission.
It should be noted that the product form of WDM is also evolving in the context of the rapid iteration of AI computing power.
With the large-scale implementation of 400G / 800G / 1.6T high-speed optical modules in AI computing centers, WDM technology has long transcended traditional external modular application scenarios and is rapidly evolving towards high integration, chip-level, and photonic integration.
For instance, for the receiving end of the 400G/800G/1.6T FR transceiver module, the industry is basically using the Z-block integrated optical path or the mature AWG discrete optical splitting structure. Looking at the transmitting side, the laser itself outputs linearly polarized light, which makes it easier for upstream to make efforts for on-chip integration - integrated wavelength division solutions like EDG, MZI Mach-Zehnder interferometer are no longer laboratory concepts but are widely used in high-speed modules
